This invention relates generally to a method and system for drilling into and through subsurface rock using high temperature aqueous solutions comprising high concentrations of hydroxyl ions.
For many years, oil and gas wells have been drilled by a rotary bit mounted on a tubular drill string extending down a borehole from the surface of the earth. The drill string is rotated at the surface, and the rotary motion is transmitted by the drill string to the bit at the bottom of the hole. A liquid commonly known as drilling mud is introduced through the drill string to cool the bit and to carry cuttings produced by the bit to the surface through the annular space between the drill string and the wall of the borehole. This method of drilling has certain limitations and advantages. The string must be relatively stiff in order to transmit torque and heavy enough to transmit sufficient axial load to the bit at the bottom of the hole. In hard rock, the drilling rate is slow, and the drill bit tends to wear rapidly. When the drill bit requires replacement, the entire string must be pulled out of the hole and broken down into tubing joints as it is removed. It is necessary to use heavy, powerful machinery to handle the relatively heavy drill string. Powerful equipment is also required in order to inject the drilling mud with sufficient pressure to remove cuttings from the bottom of the well. In horizontal drilling, removal of the cuttings by the flowing mud is particularly difficult because the axial mudflow is perpendicular to the settling direction. In the case of horizontal drilling, the weight of the drill collar can no longer be used effectively to provide the required weight on the drill bit. The horizontal distance that can be drilled becomes limited by cutting removal and by buckling of the drill string in compression unless a complex bottom hole tractor is used.
To overcome the shortcoming of rotary drilling, various means have been proposed of directing high velocity streams or jets of fluid against the material to be cut. In one approach, high-pressure fluid is discharged from the distal end of a hollow drilling tube. Hydraulic jet drill heads are typically attached to the drilling tube, with the drill heads having a multiplicity of nozzles through which the drilling fluid is discharged. Because of the difficulties in compressing fluid to very high pressures, in transporting the pressurized fluid over a long distance, and with erosion of equipment due to high velocity solids-laden fluid, the fluid jet approach of drilling boreholes has seen limited usage.
Thermal spallation is another method to drill holes through rock. Spallation, often called thermal drilling, produces thin flakes, or spalls, that flake or spall off the rock surface. In one spallation procedure, a combustion flame jet impinges on a rock surface, thereby inducing stresses high enough to cause the rock to spall. Examples of spallation drilling are disclosed in U.S. Pat. No. 5,771,984 and WO9603566. Spallation drilling has the advantage in that a drilling rig need not use rotation of the drill string. Further, there is no direct contact between the effective end of the drilling apparatus and the rock being removed, thereby avoiding wear caused by abrasion at the tool-rock interface. However, one shortcoming of spallation drilling is the difficulty of avoiding overheating of at least part of the rock to be spalled. Some types of rock will not spall if the heat flux exceeds a minimum temperature. Overheating of the rock can result in fusion of specific mineral components at the thermodynamic melting point severely impeding the spallation process. Molten rock is more resistant to spalling after resolidification and cannot easily be removed by a spallation apparatus as disclosed in U.S. Pat. No. 3,467,206.
A hybrid drilling system, wherein spallation drilling using a flame jet is combined with mechanical cutting and erosion using ultra-high pressure hydrojets, is disclosed by J. North, S. T. Knibb, and S. M. Farouq Ali in the Journal of Canadian Petroleum Technology, Volume 40, P 67, 2001. In one example of this technique, 0.075% ethanoic acid is added to the water to enhance the drilling process for limestone.
Chemical dissolution of rock using jets is yet another disclosed method to drill holes through rock as disclosed in U.S. Pat. No. 2,258,001, U.S. Pat. No. 5,964,303 and W. C. Mauer, xe2x80x9cAdvanced Drilling Techniquesxe2x80x9d, Petroleum Publishing Co., 1980. The disadvantages of the chemical approaches proposed to date are the highly toxic and corrosive nature of the chemical agents, the extreme high temperatures required and the high expense of the chemical agents.
A need therefore exists for an improved drilling system that can effectively penetrate deep, subsurface rock formations.
The invention includes a method for penetrating rock of a subsurface formation, comprising:
(a) lowering a fluid conduit into a borehole in the subsurface formation, the conduit having a top end and a lower end and adapted to heat and discharge from the lower end a stream of aqueous fluid;
(b) introducing into the top end of the fluid conduit an aqueous fluid comprising water and hydroxides of Group I elements of The Periodic Table of Elements and mixtures thereof;
c) heating said aqueous fluid to temperatures in the range of 500xc2x0 C. to 1400xc2x0 C. to provide a heated aqueous fluid;
d) discharging from the lower end of the fluid conduit the heated aqueous fluid to contact the rock of the subsurface formation and effect dissolution of the rock therefrom; and
e) removing the dissolved rock and the heated aqueous fluid from the borehole by an ascending fluid stream.
The invention also includes a drilling system for hydrothermally drilling a subterranean formation comprising a coiled tubing with an upper end and a lower end; a body attached to the lower end of the coiled tubing, said body having a heating chamber disposed therein and a plurality of nozzles opening through said body adjacent the bottom of the body so as to communicate with the heating chamber, said chamber capable of heating aqueous fluid, said nozzles capable of directing heated aqueous fluid to formation rock below the coiled tubing.
The invention further includes a drilling system for hydrothermally drilling a subterranean formation comprising a coiled tubing with an upper end and a lower end; a body attached to the lower end of the coiled tubing, said body having a combustion chamber disposed therein and having a first set of nozzles opening through the bottom of the body, said first set of nozzles being capable of emitting chemical reactants and products of combustion occurring in said combustion chamber and a second set of nozzles opening through the bottom of the body adjacent to the first set of nozzles, said second set of nozzles being capable of emitting a heated aqueous fluid comprising water and hydroxides of Group I elements of The Periodic Table of Elements and mixtures thereof and said first set and second set of nozzles capable of directing heated aqueous fluid to formation rock below the coiled tubing.